The invention relates to a method for determining a wobble component due to a wheel hub of a vehicle and to a brake disc for carrying out this method.
On account of the increased demands for comfort, wheel brake discs of motor vehicles may only have a minimum wobble; a maximum of approximately 40 xcexcm at a diameter of 200 mm.
Greater wobbling of the brake disc leads to so-called brushing wear of the brake surface at the circumferential points of the greatest axial elevation of the brake disc during driving. At these circumferential points, the brake disc then has a changed surface texture in comparison with the other circumferential points, different frictional behaviour and a slight axial decrease in thickness. When actuated, this results in so-called brake juddering, i.e. a circumferentially synchronous, periodic fluctuation of the braking action.
In this connection, this depends less on the wobble of the isolated individual part than on that of the overall assembly comprising wheel bearings, hub inner contour, hub flange and brake disc. Due to the manufacturing process, each of these parts or contours is inherently provided with an unavoidable shape and measurement inaccuracy, these inaccuracies overriding one another in the assembly. The individual inaccuracies are dispersed statistically around an average value. The shape deviations are composed diversely of surface roughness, undulatory nature of the component and wobble. When a total wobble of x xcexcm is permitted at the assembly of the axle head, consisting overall of n parts together with the brake, theoretically, for each. individual part, a wobble of x/n maximum should be permitted. It ensues that the maximum permitted wobble of the brake disc applies to its outer circumference; a correspondingly smaller wobble, in accordance with the diameter ratio of brake disc to hub flange diameter, has to be prescribed in the case of a hub flange having a substantially smaller external diameter.
In this connection, wobble means an axial, sinoid positional deviation (single-phase wobble) which reproduces itself periodically during a single revolution of a component. The problem existing here is composed of a number of factors:
For one thing, the dimensional and shape deviations which are not of interest here, such as surface roughness and undulatory nature of the component, greatly override, with regard to their tolerable extent, the wobble which is actually of interest, with the result that the parameter which is of interest here cannot be isolated by measuring technology from a scanning record. The tolerable surface roughnesses and undulatory nature of the component are greater in their extent than the wobble which can be tolerated for an individual part.
Also, during the assembly, the parts become distorted, because of the joining force, in an extent of at least the same order of magnitude as the maximum permissible individual wobble for each component or component contour, i.e. even the component warpage due to assembly greatly overrides the wobble which is actually of interest.
Also, should it actually be possiblexe2x80x94for example, because of certain filter treatmentsxe2x80x94to isolate, using measurement technology, from a scanning record which is heavily overriden by signals which are not of interest, the single-phase wobble which is of interest here, it is nevertheless not possible to determine from this that component of the wobble which is attributed to the wheel hub.
Also, the factor in avoiding the brake juddering is for the wobble of the overall constructional unit to be as small as possible. In spite of keeping the wobble at the individual parts appropriately low, it may happen that during the assembly of the individual parts, an overall wobble is produced which is higher than the sum of the individual wobbling. Also the assembly of the individual parts itself therefore likewise generates a further wobble component which is dispersive in its effect.
Also, in order to be able to ensure the replaceability of the brake disc on its ownxe2x80x94as a wearing partxe2x80x94a constructional unit formed from a wheel hub and brake disc would not be able to be machined in respect of a slight wobble in the assembled state. In that case, when the worn brake disc is used, the wheel hub would also have to be renewed, which is to be avoided.
Also, in order to be able to draw nearer to a solution to the problem, there have to be ascertained from the overall wobble arising after the assembly of the individual parts, wobble components which can be unambiguously assigned to a specific component, for example the wheel hub. Only if the isolated wobble component of the wheel hub can be ascertained within the assembly unit using measuring technology can methods be sought as to how this wobble component can be reduced using manufacturing technology.
Measurement of the wobble is not new and is known, for example, from GB 2 159 276 A. However, this publication is concerned merely with the measuring of a brake disc, so that the specimens which lie outside the tolerance limits can be ejected. The determination of a wobble component due to the wheel hub of a vehicle wheel within a multi-part overall assembly is not provided there.
It has been disclosed by JP 1-28 5453 A to individually measure the wobble of a disc rotor and of an axle stub and then, on the basis of the measurements, to, undertake a rotational alignment of the two elements with the most favourable values (-so-called matching). However, this also has nothing to do with the problem addressed above.
DE 42 25 195 A1 discloses a method for measuring radial run-out and/or axial run-out deviations of a rotor, in which the rotor, which is held in a testing machine by means of a play-compensating clamping holder, is rotated while the rotation of the rotor is scanned in the region of its circumferential and/or side surfaces by contacting or contactless sensors and a radial run-out and/or axial run-out diagram is produced therefrom, two radial run-out or axial run-out diagrams, which are rotated through 180xc2x0 with respect to one another with regard to the clamping holder, being combined with one another and geometrical measurement errors, resulting from the clamping holder, in the two diagrams being compensated for in the radial run-out and axial run-out diagram obtained by the combination.
The object of the present invention is to provide a measurement method for determining a wobble component due to the wheel hub of a vehicle wheel within a multi-part overall assembly comprising wheel bearings, hub inner contour, hub flange and brake disc. The overall assembly can therefore be measured using a known measurement method, and this measurement should then enable the wobble component due to the wheel hub to be easily ascertained. This measurement method is suitable in practice in automotive engineering. Namely, the abovementioned brake juddering originates, as mentioned, not merely from the brake disc, but also from the overall assembly, for which reason there is little point in measuring and considering the brake disc in isolation. In addition, in the case of this overall assembly generally only the brake disc is the wearing part if the substantially longer service life of the wheel bearings is left out of the equation.
Accordingly, a quite essential point of this invention is the production and use of a master brake disc which is initially fastened, for measurement purposes, to the hub instead of the brake disc actually used in the vehicle. In this case, not only are all of its values, such as surface roughness, undulatory nature, evenness and wobble, as considered over the entire angular range of 360xc2x0, known, but moreover also a precise allocation of the xe2x80x9cerrorsxe2x80x9d, with regard to their phase position on this master brake disc is known. If, using this master brake disc, the measurement values of interest of the entire assembly are determined, the known values of the master brake disc can be subtracted from the determined values at a correct phase allocation, and a differential value is then obtained which can be expressed and which constitutes the wobble of the wheel hub. Of importance here is the design of the master brake disc, in particular the master brake disc may only have noticeably smaller errors than the values which are to be measured of the hub flange and the tolerance values of this components.
The advantages of the method according to the invention and of the associated master brake disc are the simple measurement means allowing the use of a wobble-measuring machine and measurement scanning-profile device. The measurement-assisting means used, namely the master brake disc, is, as the drawing shows, in conformity with the brake disc of the vehicle, a very simple element which can therefore also be produced very precisely. Simple measurement rules are sufficient and the measurement result can be reproduced by measuring technology. Finally, a clear assignment, by measuring technology, of the result to an isolated component is also achieved in the sought-after manner.